1) Field of the Invention
The present invention relates to a device and method for estimating the temperature of an exhaust-gas purifying catalyst.
2) Description of the Related Art
In an internal combustion engine (hereinafter referred to an engine), the exhaust system is provided with an exhaust-gas purifying catalyst (hereinafter referred to simply as a catalyst) that purifies exhaust gases by causing the harmful substances in the exhaust gases to react each other to make them harmless. If this catalyst exceeds a prescribed temperature (i.e., a heat-resisting temperature), sintering (particles held by a catalytic carrier coalesce into a porous mass at high temperature) will take place. Consequently, not only does the ability to purify exhaust gases decline, but the catalyst itself also degrades thermally.
Therefore, during travel of a vehicle such as an automobile (i.e., during engine operation), it is necessary to sense or estimate the temperature of the catalyst and prevent thermal degradation of the catalyst. The temperature near the catalyst can be measured, for example, by arranging the heat-sensing portion of a wide-range temperature sensor (or a high-temperature sensor) within a catalyst container. However, since such a sensor is expensive, there have been developed techniques to estimate a catalytic temperature without providing that expensive sensor. For example, there is a technique to estimate the amount of heat radiated from the exterior surface of a catalyst from the engine cooling water temperature and vehicle speed at the time of an engine start, estimate from the amount of intake air the amount that the catalyst absorbs heat from exhaust gases, and estimate a catalytic temperature based on the amount of heat radiation and the amount of heat absorption (Japanese Patent Publication No. 2860866).
As set forth above, if the catalyst exceeds its heat-resisting temperature it will degrade thermally. The heat-resisting temperature of the catalyst is lower in an atmosphere of oxidization (lean air-fuel ratio) than in an atmosphere of deoxidization (rich air-fuel ratio). If the upper-limit temperature of the catalyst is set with a sufficient margin for the low heat-resisting temperature in an atmosphere of oxidization, and the engine is controlled so that an estimated catalytic temperature does not exceed that upper-limit temperature, then the catalyst can be reliably protected, but the engine operation is restricted greatly. Hence, in order to prevent thermal degradation of the catalyst while minimizing the restrictions on engine operating conditions, it is necessary to accurately grasp the catalytic temperature and control the engine operation so that the catalyst does not exceed its heat-resisting temperature relating to an air-fuel ratio.
Particularly, in vehicles with the function of cutting fuel during deceleration, the supply of fuel to the engine is temporarily cut in order to achieve CO2 reduction (i.e., fuel consumption reduction). Because of this, only air will be discharged from the cylinder where fuel is being cut, and the chances of the catalyst reaching high temperature in an atmosphere of oxidization will increase. Also, there are vehicles in which, when engine load is low, the air-fuel ratio is made leaner than a stoichiometric air-fuel ratio to achieve the reduction of fuel consumption. However, even in such vehicles, the chances of the catalyst reaching high temperature in an atmosphere of oxidization will increase. For that reason, a more accurate grasp of the catalytic temperature, and engine control (including air-fuel ratio control) based on that accurate temperature, are required.
However, the catalytic temperature changes with various factors, so it cannot be simply detected or estimated from the amount and temperature of exhaust gases. For instance, while exhaust gases are going from the engine to the catalyst, they are cooled by a travel wind, heat transfer to the exhaust system, etc. Also, because the catalyst itself is heated, it takes time before the temperature of the catalyst rises. For that reason, in the conventional technique for estimating a catalytic temperature solely from the amount and temperature of exhaust gases, as in the above-described Japanese Patent Publication No. 2860866, it is fairly difficult to accurately estimate a catalytic temperature.